Electrochromic (EC) materials exhibiting various colors have been successfully used for commercial products such as displays, anti-glare mirrors, sunglasses, and solar-attenuated windows. Because of relatively low driving-voltage comparing to other technique such as suspension particles or liquid crystal, electrochromic device (ECD) is attractive in terms of energy-saving and sustainability.
To enhance the coloring/bleaching contrast, a redox-complementary electrochromic device has been developed (for example, see Non Patent Literatures 1 and 2). It is a device in which a pair of electrochromic materials, i.e., anodically and cathodically coloring electrochromic materials, change their colors in phase, and thus enhancing the coloring/bleaching contrast.
One of the issues in the field of the ECD is to develop a black-to-colorless transmissive device for versatile applications. However, there are only a limited number of EC materials that show black color, among which Co-based metallo-supramolecule, which may hereinafter be referred to as PolyCo, attracts attention. PolyCo has been successfully demonstrated to switch between transmissive orange and black color, mainly attributed to the d-d transition of the cobalt ion between its Co(II) and Co(I) states induced electrochemically (for example, see Non Patent Literature 3).